The mitigation of corrosion in steam generating systems is vital to the continued efficient operation of the system. The formation of metal oxides on metal surfaces, particularly iron oxide, results in reduced heat transfer rates. These insulating deposits result in an increased metal temperature which, over time, result in creep failure of the metal. Unfortunately, these deposits are also somewhat porous which can lead to evaporative concentration of other salts within the deposit-undergoing wick boiling. The concentration effect can then result in localized attack of the metal surface beneath said porous deposits. Such attack will dissolve or gouge out a portion of the metal surface, lowering the yield and rupture strength, thus leading to rupture or fracture type failures of the metal under going heat transfer. In order to avoid such failures, steam generators must be maintained free of corrosion and other deposits by 1) minimizing corrosion in the system and/or 2) minimizing the deposition of the corrosion products found in the feedwater of the thermal system.
One means of minimizing the deposition of corrosion-forming solids is by dispersing the solids, such as corrosion products, that are found in feedwater thus avoiding fouling of heat input surfaces found in thermal system cycle. Over the years, many chemicals have been used for dispersing corrosion-forming solids in the feedwater. Early examples were inorganic phosphates, naturally derived and modified lignin, tannins, and carboxymethyl cellulose.
Later, synthetic polymers were developed to improve upon these modified natural derivative dispersants. One of the first generations of synthetic polymers was homopolymers, such as polyacrylamide, polyacrylate, and polymethylacrylate. Copolymers, terpolymers, and quadrapolymers were also developed with different monomeric functional groups wherein the ratios of these monomers and the average molecular weights of the final polymer were varied. The dispersant properties of these homopolymers, copolymers, terpolymers, and quadrapolymers are due to the use of anionic functionalities, nonionic functionalities or both (anionic/nonionic) functionalities into their backbone. The anionic monomer functionality can be derived from carboxylates, sulfonates, phosphonates, phosphinate, amido or acrylamide containing groups. The nonionic monomer functionality can be derived from vinyl acetate, ethylacrylate, tertbutylacrylamide, isobutylene, ethylene glycol, ethoxylate, alkyl, or aryl containing groups.
Examples of these dispersants include, hydrolyzed or partially hydrolyzed acrylamides/acrylates, hydroxypropylacrylate, phosphino carboxylates, phosphonate functional polymers, polymaleates, sulfonated styrene/maleic anhydride copolymers, polycarboxylated phosphonates, acrylate/acrylamide copolymers, acrylate/maleic copolymers, acrylamido methyl propane sulfonate/acrylate (AMPS/AA) copolymer, maleic/isobutylene copolymer isopropenylphosphonic/alkyl or aryl copolymer, acrylic/maleic/AMPS terpolymer, acrylic/AMPS/t-butylacrylamide terpolymer, acrylic/maleic/vinyl acetate terpolymer, acrylic/methacrylic/t-butyl acrylamide terpolymer, acrylic/AMPS/styrene sulfonate terpolymer, acrylic/AMPS/phosphinate terpolymer, ethoxylated aryl sulfonate/alkyl sulfonate and acrylic acid quadrapolymer, hydroxylamine substituted polyacrylate/acrylamide copolymers, hydrazine substituted polyacrylate polymers and nonionic polymers used in steam generating systems
Around the same time synthetic polymers were being investigated as solids dispersants, low molecular weight phosphonates were also being developed as solids dispersants. Examples of such phosphonates include aminotrimethylene phosphonic acid, hydroxyethylidene diphosphonic acid, phosphonobutane tricarboxylic acid, hexamethylene diamine tetramethylene phosphonic acid, diethylene triamine pentamethylene phosphonic acid, bis(hexamethylene) triamine phosphonic acid, ethanolamine diphosphonic acid, diaminocyclohexane tetrakis (methylene phosphonic acid), methylpentane diamine tetrakis (methylene phosphonic acid), bis(hexamethylene) triamine phosphonic acid, phosphono succinic acid, phosphono tartaric acid, and phosphono glutaric acid.
It is known to use methyl ethyl ketoxime (MEKO) as an oxygen scavenger and metal passivator in boilers. See, for instance, U.S. Pat. No. 4,487,745. This patent indicates that the amount of oxime used in treating boiler water is from 0.0001 ppm to 500 ppm, although commercial utility plant experience indicates that the typical dosage of MEKO used to control feedwater oxygen scavenging is from 30-80 ppb. MEKO controls corrosion in the feedwater circuit by scavenging oxygen and by establishing a corrosion-resistant oxide film on waterside metallic surfaces. In several cases, sodium polymethacrylate was present in the boiler to minimize the deposition of corrosion-forming solids, and MEKO was present in the boiler to scavenge oxygen.